1. Field of the Invention
The present invention relates to ink jet printing apparatus of the continuous type and more specifically to features of a simplified ink circulation system (e.g. including an ink supply cartridge and ink circulation means) in such printing apparatus.
2. Description of the Prior Art
In continuous ink jet printing apparatus streams of uniformly spaced ink drops are created by imposing predetermined vibrations upon liquid ink filaments issuing from an orifice plate. The filaments are formed by supplying ink under pressure to a print head cavity that is in communication with the orifice plate. Information is imparted to the droplet streams by selective non-charging or charging and deflection of droplets. A portion of the droplets pass to the recording medium but there are a substantial number of non-printing droplets which are intercepted by a catcher for recirculation. Often the print head cavity has an outlet other than the orifice plate (e.g. to facilitate dynamic pressure control within the cavity at start-up), and the apparatus ink supply system also circulates such ink flow.
The uniformity of the stream of ink droplets is dependent upon maintaining the orifice plate structure clear of obstructions, e.g. unwanted particles ink the ink, and upon a uniform pressure in the print head cavity; and these are important concerns in the ink supply system. It is also important that recirculating ink be returned to the supply reservoir reliably. For example, if ink intercepted by the catcher is not reliably withdrawn to the ink supply, an accumulation of ink in the catcher region can impede the path of printing drops.
Heretofore the ink circulation systems in continuous ink jet printing apparatus have been fairly complex and large in size. A typical approach was to provide a supply pump, under the control of upstream detection and feedback, to assure proper dynamic print head pressure and to employ a separate vacuum pump for drawing ink back to the supply reservoir (e.g. from the catcher or print head outlet).
Concurrently filed U.S. patent application Ser. No. 06/722,550, entitled "Ink Circulation System for Continuous Ink Jet Printing Apparatus" discloses a simplified ink circulation approach which provides a vented ink reservoir and means located along the ink circulation path for creating, from the dynamics of the ink flow, a reduced pressure region(s) to withdraw ink from the catcher and/or print head outlet. This unique approach avoids the ink evaporation (and resulting viscosity change) caused by a vacuum pump continuously withdrawing air over the ink in a supply reservoir. This approach also obviates the need for a separate vacuum pump and the additional cost, complexity, size and energy usage incident thereto.